When I flush the toilet, water comes out of the drain hole from the water heater

When I flush the toilet, water comes out of the drain hole from the water heater

Sounds like you have a closed system and air chambers that need recharging. I would install a expansion tank,(see image) on the heaters cold water supply. Do you have any water hammer when you close a faucet. Does closing a faucet cause the heaters T & P valve to leak water. Let me know, n Tom